Secondary controller for use in synchronous flyback converter

ABSTRACT

A secondary controller for use in a power converter includes a drive circuit coupled to a secondary side of the power converter. The drive circuit is coupled to generate a first signal to enable a first switch coupled to a primary side of the primary converter. The first signal is generated in response to a feedback signal representative of an output of the power converter. A control circuit is coupled to receive the first signal and an input signal representative of a secondary winding voltage of the power converter. The control circuit is coupled to generate a second signal to control a second switch coupled to the secondary side of the power converter in response to the first signal and the input signal.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/746,776, filed on Jan. 22, 2013, now pending. U.S. patent applicationSer. No. 13/746,776 is hereby incorporated by reference.

BACKGROUND INFORMATION

Field of the Disclosure

The present invention relates generally to controlling a powerconverter. More specifically, examples of the present invention arerelated to controlling switch mode power converters.

Background

Switch mode power converters are widely used for household or industrialappliances that require a regulated direct current (dc) source for theiroperation, such as for example battery chargers that are commonly usedin electronic mobile devices. Off-line ac-dc converters convert a lowfrequency (e.g., 50 Hz or 60 Hz) high voltage ac (alternating current)input voltage to a required level of dc output voltage. Various types ofswitch mode power converters are popular because of their well regulatedoutput, high efficiency, and small size along with their safety andprotection features. Popular topologies of switch mode power convertersinclude flyback, forward, boost, buck, half bridge and full bridge,among many others including resonant types.

Some switch mode power converters, such as a synchronous switch modepower converter, may include a first switch on the primary side of thepower converter and also a second switch, such as a switch of asynchronous rectification circuit, on the secondary side of the powerconverter. The first switch may be switched between an ON state (i.e.,closed switch) and an OFF state (i.e., open switch) to control theenergy transfer between the input and the output of the power converter.The second switch may be used to increase the efficiency with which theenergy is transferred to the output of the power converter when thefirst switch is switched to the OFF state. In operation, the secondswitch may be switched between the ON state and the OFF state incoordination with the first switch such that both switches are not in ONstate simultaneously to prevent a condition where the power converterattempts to provide energy to a short circuit at the output that maylead to a reduction in the efficiency of the power converter.

A synchronous switch mode power converter may operate in both acontinuous conduction mode and a discontinuous conduction mode dependingon a load condition at the output of the power converter. It may bedesirable for the power converter to run efficiently in both operationmodes. Therefore, the secondary side of the power converter maycoordinate the control of the second switch with the primary side of thepower converter to ensure that the first switch and the second switchare not in ON state at the same time in both the continuous conductionmode and the discontinuous conduction mode.

BRIEF DESCRIPTION OF THE DRAWINGS

Non-limiting and non-exhaustive embodiments of the present invention aredescribed with reference to the following figures, wherein likereference numerals refer to like parts throughout the various viewsunless otherwise specified.

FIG. 1 shows a schematic of one example of a synchronous flyback switchmode power converter that includes a primary controller coupled toreceive a signal from a secondary controller of the synchronous flybackswitch mode power converter, in accordance with the teachings of thepresent invention.

FIG. 2 shows a block diagram schematic of an example control circuitthat may be included in a secondary controller of a synchronous flybackswitch mode power converter, in accordance with the teachings of thepresent invention.

FIG. 3A illustrates an example timing diagram showing signals associatedwith a secondary controller operating in discontinuous conduction mode,in accordance with the teachings of the present invention.

FIG. 3B illustrates an example timing diagram showing signals associatedwith a secondary controller operating in continuous conduction mode, inaccordance with the teachings of the present invention.

FIG. 4 is a flow chart illustrating an example process of operation of asynchronous flyback switch mode power converter, in accordance with theteachings of the present invention.

Corresponding reference characters indicate corresponding componentsthroughout the several views of the drawings. Skilled artisans willappreciate that elements in the figures are illustrated for simplicityand clarity and have not necessarily been drawn to scale. For example,the dimensions of some of the elements in the figures may be exaggeratedrelative to other elements to help to improve understanding of variousembodiments of the present invention. Also, common but well-understoodelements that are useful or necessary in a commercially feasibleembodiment are often not depicted in order to facilitate a lessobstructed view of these various embodiments of the present invention.

DETAILED DESCRIPTION

In the following description, numerous specific details are set forth inorder to provide a thorough understanding of the present invention. Itwill be apparent, however, to one having ordinary skill in the art thatthe specific detail need not be employed to practice the presentinvention. In other instances, well-known materials or methods have notbeen described in detail in order to avoid obscuring the presentinvention.

Reference throughout this specification to “one embodiment”, “anembodiment”, “one example” or “an example” means that a particularfeature, structure or characteristic described in connection with theembodiment or example is included in at least one embodiment of thepresent invention. Thus, appearances of the phrases “in one embodiment”,“in an embodiment”, “one example” or “an example” in various placesthroughout this specification are not necessarily all referring to thesame embodiment or example. Furthermore, the particular features,structures or characteristics may be combined in any suitablecombinations and/or subcombinations in one or more embodiments orexamples. Particular features, structures or characteristics may beincluded in an integrated circuit, an electronic circuit, acombinational logic circuit, or other suitable components that providethe described functionality. In addition, it is appreciated that thefigures provided herewith are for explanation purposes to personsordinarily skilled in the art and that the drawings are not necessarilydrawn to scale.

FIG. 1 shows a schematic of one example of a synchronous flyback switchmode power converter 100 that includes a primary controller 180 coupledto receive a signal from a secondary controller 140 of the synchronousflyback switch mode power converter 100, in accordance with theteachings of the present invention. In the example illustrated in FIG.1, synchronous flyback switch mode power converter 100 utilizessecondary control. It is appreciated that secondary control for aflyback converter has advantages of tighter output regulation and fasterresponse to load transients.

Synchronous flyback switch mode power converter 100 receives anunregulated input voltage V_(IN) 102 at the input to produce an outputvoltage V_(O) 120 and an output current I_(O) 118 to an electrical load128. Input voltage V_(IN) 102 may be a rectified and filtered acvoltage. As shown, input voltage V_(IN) 102 is referenced to a primaryground 104, also referred to as an input return and output voltageV_(OUT) 120 is referenced to a secondary ground 122, also referred to asan output return. In other examples, synchronous flyback switch modepower converter 100 may have more than one output.

As further shown in FIG. 1, synchronous flyback switch mode powerconverter 100 includes a primary controller 180 and a secondarycontroller 140 to control the circuits of power converter 100 toregulate the output voltage V_(O) 120 at a desired voltage level. In oneexample, synchronous flyback switch mode power converter 100 mayregulate output voltage V_(O) 120 to the desired voltage level after astartup period. The startup period may be a period of time starting fromwhen synchronous flyback switch mode power converter 100 is introducedto input voltage V_(IN) 102 until primary controller 180 and secondarycontroller 140 begin operating to regulate output voltage V_(O) 120. Inthe example synchronous flyback switch mode power converter 100, anoutput capacitor C1 116 is coupled to the output to smooth out theripple in output voltage V_(O) 120.

Also included in FIG. 1 is an energy transfer element T1 124 that isillustrated as a coupled inductor with a primary winding 110 and asecondary winding 112. Energy transfer element T1 124 is coupled totransfer energy from primary winding 110 to secondary winding 112.Additionally, energy transfer element T1 124 provides galvanic isolationbetween circuits on the primary side of synchronous flyback switch modepower converter 100 and circuits on the secondary side of synchronousflyback switch mode power converter 100. In other words, a dc voltageapplied between the primary side and the secondary side of synchronousflyback switch mode power converter 100 will produce substantially zerocurrent.

Circuits that are electrically coupled to primary winding 110 may bereferred to as the primary side of synchronous flyback switch mode powerconverter 100. Similarly, circuits that are electrically coupled tosecondary winding 112 may be referred to as the secondary side ofsynchronous flyback switch mode power converter 100. In the depictedexample, a switching device S1 150 is coupled to energy transfer element124 at primary winding 110 and coupled to the input of synchronousflyback switch mode power converter 100 at input return 104. Switchingdevice S1 150 may be a metal oxide field effect transistor (MOSFET), abipolar junction transistor (BJT) or the like. As shown, primarycontroller 180 is coupled to the circuit components of the primary sidesuch as switching device S1 150. Secondary controller 140 is coupled tothe circuits on the secondary side such as a synchronous rectificationcircuit 126, secondary winding 112 along with other circuit components.In operation, primary controller 180 and secondary controller 140control the circuits of synchronous flyback switch mode power converter100 (e.g., switching device S1 150 and synchronous rectification circuit126) to control energy transfer through energy transfer element T1 124from the input to the output of synchronous flyback switch mode powerconverter 100.

A clamp circuit 106 is coupled across primary winding 110 of energytransfer element T1 124 and to the input of synchronous flyback switchmode power converter 100. Clamp circuit 106 operates to clamp anyturn-off spikes that result from leakage inductance from primary winding110 across the switching device S1 150.

Primary controller 180 and secondary controller 140 may be included inan integrated circuit. In one example, primary controller 180 isincluded in a first integrated circuit die and a secondary controller140 is included in a second integrated circuit die that are bothdisposed in an integrated circuit package. In one example, switchingdevice S1 150 may be included in a monolithic or hybrid structure in anintegrated circuit package that also includes the primary controller 180and the secondary controller 140. In one example, switching device S1150 is disposed on a first integrated circuit die that also includesprimary controller 180 and secondary controller 140 is included in asecond integrated circuit die. In another example, switching device S1150 is disposed on a first integrated circuit die, primary controller180 is included in a second integrated circuit die, and secondarycontroller 140 is included in a third integrated circuit die. The dieincluding the primary controller 180 is galvanically isolated from thedie including the secondary controller 140. Accordingly, primarycontroller 180 is galvanically isolated from secondary controller 140.

Although primary controller 180 and secondary controller 140 aregalvanically isolated from one another, primary controller 180 andsecondary controller may communicate with one another. Specifically,secondary controller 140 may communicate with primary controller 180 byproviding a signal through a magnetically coupled communication link(not shown in FIG. 1). In one example, the communication link betweenprimary controller 180 and secondary controller 140 may be implementedusing galvanically isolated conductive loops included in the lead frameof an integrated circuit package. Alternatively, secondary controller140 may provide a signal to primary controller 180 through anopto-coupler or a coupled inductor.

In the illustrated example, secondary controller 140 transmits a delayeddrive signal U_(DPD) 148 to primary controller 180. Primary controller180 controls the state of switching device S1 150 in response to delayeddrive signal U_(DPD) 148. For example, switching device S1 150 mayoperate in an ON state (e.g., as a closed switch) or in an OFF state(e.g., as an open switch) depending on switch drive signal 182 generatedby primary controller 180 in response to delayed drive signal U_(DPD)148. In operation, primary controller 180 controls the primary currentI_(SW) 130 through switching device S1 150 and primary winding 110. Inthe illustrated example, primary controller 180 senses primary currentI_(SW) 130 as current sense signal 134. To sense primary current I_(SW)130, a variety of techniques may be used including receiving voltageacross a resistor conducting current, receiving a scaled current from acurrent transformer, receiving the voltage across an on-resistance of aMOSFET that conducts current, or otherwise.

The waveform of current sense signal 134 in FIG. 1 shows thatsynchronous flyback switch mode power converter 100 is operating indiscontinuous conduction mode, which may be typical for operation atlight loads. A distinguishing characteristic of operation indiscontinuous conduction mode is that primary current I_(SW) 130 issubstantially zero shortly after switching device S1 150 turns ON. Athigher loads, synchronous flyback switch mode power converter 100typically operates in continuous conduction mode, which is distinguishedby having primary current I_(SW) 130 having a non-zero value shortlyafter switching device S1 150 turns ON.

When switching device S1 150 is ON, the current through primary winding110 increases the energy stored in energy transfer element T1 124. Aprimary winding voltage V_(P) 108 with a first polarity develops acrossprimary winding 110. A secondary winding voltage V_(S) 113 with anopposite polarity of V_(P) 108 develops across secondary winding 112when switching device S1 150 is in ON state. Synchronous rectificationcircuit 126 may act as an open circuit when secondary winding voltageV_(S) 113 is a positive voltage.

Primary controller 180 may transition switching device S1 150 from theON state to the OFF state, thereby blocking the current throughswitching device S1 150 when primary current I_(SW) 130 reaches acurrent limit I_(P) 132, which in one example is a fixed limit. Whenswitching device S1 150 transitions from the ON state to the OFF state,secondary winding voltage V_(S) 113 becomes a negative voltage andallows for energy to be transferred to output capacitor C1 116,providing power to electrical load 128. In one example, secondarycontroller 140 may control synchronous rectification circuit 126 to actas a closed switch (i.e., to conduct current) when secondary windingvoltage V_(S) 113 transitions to a negative voltage so that outputcapacitor C1 116 is charged.

In the example, synchronous flyback switch mode power converter 100 alsoincludes a secondary winding sense circuit 168. In one example,secondary winding sense circuit 168 is coupled to sense a forwardvoltage V_(F) 167 representative of secondary winding voltage V_(S) 113and to generate a clamped version of forward voltage V_(F) 167. In oneexample, forward voltage V_(F) 167 is substantially equal to the sum ofoutput voltage Vo 120 and secondary winding voltage V_(S) 113. In theillustrated example, secondary winding sense circuit 168 generates aclamped forward voltage V_(CF) 166, representative of the clampedversion of forward voltage V_(F) 167, by limiting the positive amplitudeof clamped forward voltage V_(CF) 166. More specifically, secondarywinding sense circuit 168 includes a high voltage n-channel MOSFET,which is coupled to secondary winding 112, to sense forward voltageV_(F) 167 and to produce clamped forward voltage V_(CF) 166 by limitingthe positive amplitude of clamped forward voltage V_(CF) 166 toapproximately 4.3V. It is noted that in other examples, secondarywinding sense circuit 168 may not necessarily limit the positiveamplitude of the signal generated at the output of secondary windingsense circuit 168. Secondary winding sense circuit 168 is enabled whensynchronous flyback switch mode power converter 100 enters normaloperation after secondary controller 140 is powered up during startup ofsynchronous flyback switch mode power converter 100. In one example,secondary winding sense circuit 168 may be disposed on the sameintegrated circuit die as secondary controller 140. In another example,secondary winding sense circuit 168 may be located outside theintegrated circuit die that includes secondary controller 140 and may becoupled to provide clamped forward voltage V_(CF) 166 to a winding senseterminal of secondary controller 140.

In the depicted example, secondary controller 140 includes a drivecircuit 144 to generate a drive signal U_(PD) 146 in response to afeedback signal U_(FB) 154, which is representative of an outputquantity U_(O) 156 of synchronous flyback switch mode power converter100. Output quantity U_(O) 156 may include output voltage V_(O) 120and/or output current I_(O) 118. In one example, a sense circuit 152 iscoupled to sense output quantity U_(O) 156 and to generate feedbacksignal U_(FB) 154 in response to output quantity 156. In the illustratedexample, secondary controller 140 is coupled to sense circuit 152 toreceive feedback signal U_(FB) 154 at a feedback terminal of secondarycontroller 140. In one example, sense circuit 152 includes a resistivedivider coupled to the output of synchronous flyback switch mode powerconverter 100 to generate feedback signal U_(FB) 154 as a scaled downvoltage representative of output voltage V_(O) 120. In one example,sense circuit 152 is disposed on the same integrated circuit die assecondary controller 140.

In the example secondary controller 140, drive signal U_(PD) 146 may berepresentative of an enabled switching period or a disabled switchingperiod for switching device S1 150. In other words, drive signal U_(PD)146 may indicate whether primary controller 180 should enable (allow toturn ON) or disable (not allow to turn ON) switching device S1 150 in aswitching period which is defined by the time period between consecutivepulses of a clock signal U_(CLK) 172 generated by an oscillator 170 thatis included in secondary controller 140. For example, drive signalU_(PD) 146 may indicate that primary controller 180 should enableswitching device S1 150 to provide more energy to the secondary side ofpower converter 100 in a switching period when output voltage V_(O) 120drops below a desired voltage level. Similarly, drive signal U_(PD) 146may indicate that primary controller 180 should disable switching deviceS1 150 to provide less energy to the secondary side of power converter100 in a switching period when output voltage V_(O) 120 is substantiallyequal to or greater than the desired voltage level.

In the illustrated example, drive circuit 144 is coupled to receiveclock signal U_(CLK) 172 from oscillator 170. In one example, drivecircuit 144 compares feedback signal U_(FB) 154 to an output thresholdV_(TH) and generates drive signal U_(PD) 146 as a logic high signal iffeedback signal U_(FB) 154 is less than the output threshold V_(TH) whenclock signal U_(CLK) 172 is logic high. Drive circuit 144 generatesdrive signal U_(PD) 146 as a logic low signal if feedback signal U_(FB)154 is greater than the output threshold V_(TH) and/or clock signalU_(CLK) 172 is logic low. In the illustrated example, drive circuit 144includes an AND-gate coupled to output drive signal U_(PD) 146 inresponse clock signal U_(CLK) 172 and in response to the comparison offeedback signal U_(FB) 154 to the output threshold V_(TH). As furtherillustrated, the AND-gate is a two input AND-gate. Drive circuit 144also includes an inverter coupled between a first input of the AND-gateand a comparator. In one example, the comparator is a voltage comparatorwith a threshold voltage substantially equal to the output thresholdV_(TH) and is further coupled to receive feedback signal U_(FB) 154. Asecond input of the AND-gate is coupled to receive clock signal U_(CLK)172 from oscillator 170.

As further illustrated in FIG. 1, secondary controller 140 also includesa delay circuit 162 and a control circuit 160. Delay circuit 162 iscoupled to drive circuit 144 to receive drive signal U_(PD) 146 andcoupled to delay drive signal U_(PD) 146 to generate delayed drivesignal U_(DPD) 148. In the illustrated example, control circuit 160 iscoupled to receive drive signal U_(PD) 146, delayed drive signal U_(DPD)148, and clamped forward voltage V_(CF) 166. Control circuit 160 iscoupled to generate a control signal U_(CR) 164 in response to drivesignal U_(PD) 146 and in response to clamped forward voltage V_(CF) 166,as shown. Control circuit 160 may also be coupled to generate controlsignal U_(CR) 164 in response to delayed drive signal U_(DPD) 148.

Control signal U_(CR) 164 controls synchronous rectification circuit126. As shown in the example of FIG. 1, synchronous rectificationcircuit 126 is coupled to secondary winding 112 on the secondary side ofsynchronous flyback switch mode power converter 100. In the illustratedexample, synchronous rectification circuit 126 includes a switch S2 127controlled by control signal U_(CR) 164 from secondary controller 140.In one example, switch S2 127 is a MOSFET whose gate is coupled tocontrol signal U_(CR) 164. Switch S2 127 may operate in the ON state(i.e., switch S2 127 is turned ON) or in the OFF state (i.e., switch S2127 is turned OFF) depending on control signal U_(CR) 164. When turnedON by the control signal U_(CR) 164 from secondary controller 140,switch S2 127 of synchronous rectification circuit 126 may conductcurrent. In the illustrated example, synchronous rectification circuit126 includes a diode, which may be a discrete component or may beincluded in the same component as the illustrated switch (e.g., bodydiode of the MOSFET).

Control circuit 160 may control switch S2 127 of synchronousrectification circuit 126 such that switching device S1 150 and switchS2 127 are not in ON state simultaneously which, if happens, may greatlyreduce the efficiency of synchronous flyback switch mode power converter100. Control circuit 160 may monitor clamped forward voltage V_(CF) 166and turn ON switch S2 127 when clamped forward voltage V_(CF) 166becomes a negative voltage indicating that switching device S1 150 hastransitioned to OFF state. In one example, control circuit 160 maycompare clamped forward voltage V_(CF) 166 to a negative thresholdvoltage to determine if clamped forward voltage V_(CF) 166 has become anegative voltage.

When switch S2 127 is in ON state, the energy stored in energy transferelement T1 124 is transferred to the output of synchronous flybackswitch mode power converter 100 with a secondary current (not shown)charging output capacitor C1 116. The secondary current may decrease asoutput voltage V_(O) 120 increases (i.e., as more energy is transferredto the output of synchronous flyback switch mode power converter 100).Since the current in switch S2 127 is substantially the same as thesecondary current, the voltage drop across switch S2 127 may alsodecrease. This results in forward voltage V_(F) 167 (and clamped forwardvoltage V_(CF) 166) becoming less negative with respect to output return122, which in one example is substantially zero volts. In one example,control circuit 160 may turn OFF switch S2 127 when clamped forwardvoltage V_(CF) 166 becomes substantially zero volts.

When synchronous flyback switch mode power converter 100 operates in thediscontinuous conduction mode, the secondary current drops tosubstantially zero before the start of the next enabled switching period(i.e., before drive signal U_(PD) 146 becomes a logic high signal).Accordingly, forward voltage V_(F) 167 (and clamped forward voltageV_(CF) 166) increases to substantially zero volts from the negativevoltage. Therefore, in the discontinuous conduction mode, controlcircuit 160 may use clamped forward voltage V_(CF) 166 to determine whento turn OFF switch S2 127. Specifically, control circuit 160 may compareclamped forward voltage V_(CF) 166 to the same negative thresholdvoltage and turn OFF switch S2 127 if clamped forward voltage V_(CF) 166is substantially equal to or greater than the negative thresholdvoltage.

When synchronous flyback switch mode power converter 100 operates in thecontinuous conduction mode, the secondary current does not drop tosubstantially zero before drive signal U_(PD) 146 indicates that primarycontroller 180 should enable switching device S1 150 (i.e., before thestart of the next enabled switching period). Accordingly, forwardvoltage V_(F) 167 (and clamped forward voltage V_(CF) 166) may staylower than the negative threshold voltage until switching device S1 150transitions to the ON state. As a result, control circuit 160 may notuse clamped forward voltage V_(CF) 166 to turn OFF switch S2 127 beforeswitching device S1 150 transitions to ON state. In the illustratedexample, in continuous conduction mode, control circuit 160 turns OFFswitch S2 127 when drive signal U_(PD) 146 becomes a logic high signalindicating that primary controller 180 should enable switching device S1150. It should be noted that although drive signal U_(PD) 146 indicatesthat primary controller 180 should enable switching device S1 150,primary controller 180 does not transition switching device S1 150 tothe ON state before receiving the delayed version of drive signal U_(PD)146 (i.e., delayed drive signal U_(DPD) 148) from secondary controller140. It is in this manner that control circuit 160 ensures thatswitching device S1 150 and switch S2 127 are not in the ON state at thesame time in both the discontinuous conduction mode and the continuousconduction mode of operation.

In the illustrated example, secondary controller 140 may be coupled toreceive power from the secondary side of the synchronous flyback switchmode power converter 100. For example, secondary controller 140 may becoupled to a bypass capacitor (not shown) which may be coupled tosecondary winding 112. When charged to a certain voltage level, thebypass capacitor may provide power to operate the circuits of secondarycontroller 140 such as control circuit 160. At startup, e.g., when inputvoltage V_(IN) 102 is introduced to the input of synchronous flybackswitch mode power converter 100, primary controller 180 starts switchingthe state of switching device S1 150 between the ON state and the OFFstate and hence, the energy transfer to the secondary side ofsynchronous flyback switch mode power converter 100. However, thesecondary side of synchronous flyback switch mode power converter 100may not yet provide sufficient power to secondary controller 140 becausefor example, the bypass capacitor may be uncharged or may be charged toa voltage level lower than the required minimum to operate secondarycontroller 140. Therefore, secondary controller 140 may not send delayeddrive signal U_(DPD) 148 to primary controller 180 and may not producecontrol signal U_(CR) 164 to control switch S2 127 of synchronousrectification circuit 126. Accordingly, at startup, switch S2 127 mayremain in the OFF state and primary controller 180 may control the stateof switching device S1 150 without receiving delayed drive signalU_(DPD) 148 from secondary controller 140. It should be noted thatalthough switch S2 127 may remain in the OFF state during the startup,the diode of synchronous rectification circuit 126 may conduct currentwhen secondary winding voltage V_(S) 113 becomes a negative voltage andhence, may allow for energy transfer from the primary side to thesecondary side of synchronous flyback switch mode power converter 100.In this manner, the energy in the secondary side of synchronous flybackswitch mode power converter 100 may increase during the startup andreach a level that is sufficient to operate secondary controller 140.

FIG. 2 shows a block diagram schematic of an example control circuit 260that may be used as control circuit 160 in a secondary controller of ansynchronous flyback switch mode power converter, in accordance with theteachings of the present invention. In the illustrated example, controlcircuit 260 includes a comparator 250, logic circuitry 270, and aone-shot circuit 258. Comparator 250 is coupled to generate a comparesignal U_(CMP) 252 in response to a comparison of a secondary thresholdV_(TN) to an input signal (e.g., clamped forward voltage V_(CF) 166)representative of a secondary winding voltage (e.g., secondary windingvoltage V_(S) 113) of a synchronous flyback switch mode power converter.In one example, the secondary threshold V_(TN) is −30 mV. Although FIG.2 shows comparator 250 implemented as a voltage comparator, alternativeexamples may include a current comparator, or otherwise.

In FIG. 2, control circuit 260 also includes logic circuitry 270 coupledto comparator 250 and coupled to receive compare signal U_(CMP) 252.Logic circuitry 270 is also coupled to receive drive signal U_(PD) 146from drive circuit 144. Logic circuitry 270 may be coupled to generatecontrol signal U_(CR) 164 to control switch S2 127 of synchronousrectification circuit 126 in response to compare signal U_(CMP) 252 anddrive signal U_(PD) 146.

In the illustrated example, logic circuitry 270 includes a latch 256 andlatch 256 is coupled to be set in response to drive signal U_(PD) 146.Logic circuitry 270 may be coupled to receive delayed drive signalU_(DPD) 148 from delay circuit 162 coupled to generate delayed drivesignal U_(DPD) 148 by delaying drive signal U_(PD) 146. Latch 256 iscoupled to be set by delayed drive signal U_(DPD) 148, in theillustrated example. When latch 256 is set, the output of the latchgenerates enable signal U_(SREN) 254 that goes to logic high. In theillustrated example, logic circuitry includes an AND-gate coupled togenerate control signal U_(CR) 164 (at the AND-gate output) in responseto drive signal U_(PD) 146, in response to compare signal U_(CMP) 252,and in response to enable signal U_(SREN) 254 from the latch output. Inthe illustrated example, the AND-gate is a three input AND-gate. Logiccircuitry 270 also includes an inverter coupled between comparator 250and a first input to the AND-gate, in the illustrated example.Additionally, an inverter is coupled to a second input of the AND-gateand coupled to invert drive signal U_(PD) 146. A third input of theAND-gate receives enable signal enable signal U_(SREN) 254, in theillustrated example.

Still referring to FIG. 2, a one-shot circuit 258 is coupled to receivecontrol signal U_(CR) 164 and reset latch 256 in response to controlsignal U_(CR) 164. In one example, one-shot circuit 258 is coupled toreset latch 256 in response to a falling edge of control signal U_(CR)164. In one example, in response to control signal U_(CR) 164, one-shotcircuit 258 generates a pulse as one-shot signal U_(ONE) 257 to resetlatch 256. After latch 256 is reset, enable signal U_(SREN) 254 on thelatch output goes to logic low.

FIG. 3A illustrates an example timing diagram showing signals associatedwith a secondary controller (e.g., secondary controller 140) operatingin the discontinuous conduction mode, in accordance with the teachingsof the present invention. Clock signal U_(CLK) 310 is one possiblerepresentation of clock signal U_(CLK) 172. Feedback signal U_(FB) 320is one possible representation of feedback signal U_(FB) 154. Drivesignal U_(PD) 346 and delayed drive signal U_(DPD) 348 are one possiblerepresentation of drive signal U_(PD) 146 and delayed drive signalU_(DPD) 148, respectively. Clock signal U_(CLK) 310 is a periodic pulsewhose period represents a switching period T_(SW) (between time t₁ andtime t₅). However, the actual switching period of synchronous flybackswitch mode power converter 100 may be different from switching periodT_(SW) because drive signal U_(PD) 346 determines whether synchronousflyback switch mode power converter 100 has an enabled switching periodor a disabled switching period. An enabled switching period represents aswitching period where switching device S1 150 is allowed to switch fromthe OFF state to the ON state (i.e., switching device S1 150 is enabled)in response to the switch drive signal 182. On the other hand, adisabled switching period represents a switching period where switchingdevice S1 150 is not allowed (i.e., switching device S1 150 is disabled)to switch from the OFF state to the ON state and hence, remains in theOFF state. Specifically, if drive signal U_(PD) 346 is logic high in aswitching period T_(SW), switching device S1 150 is then enabled in thatswitching period T_(SW). However, if drive signal U_(PD) 346 is logiclow in a switching period T_(SW), switching device S1 150 is disabled inthat switching period T_(SW). In an enabled switching period, primarycontroller 180 sets the switch drive signal 182 to logic high uponreceiving a pulse in delayed drive signal U_(DPD) 348 and as a result,switching device S1 150 may transition from the OFF state to the ONstate. In a disabled switching period, primary controller 180 does notreceive a pulse in delayed drive signal U_(DPD) 348 and holds the switchdrive signal 182 at logic low, thereby causing switching device S1 150to remain in the OFF state.

The time period between time t₁ and time t₅ shows an enabled switchingperiod, in FIG. 3A. The time period between time t₅ and time t₆ shows adisabled switching period. The time period after time t₆ shows the startof another enable switching period.

Between time t₁ and time t₂, clock signal U_(CLK) 310 goes from logiclow to logic high for a short period while feedback signal U_(FB) 320 isbelow the output threshold V_(TH). Since this is an indication thatoutput voltage V_(O) 120 is below a desired output voltage level, drivecircuit 144 may switch drive signal U_(PD) 346 to logic high for aduration that is substantially equal to the pulse width of the pulse ofclock signal U_(CLK) 310. Still referring to the time period betweentime t₁ and time t₂, comparator 250 outputs compare signal U_(CMP) 352,one possible representation of compare signal U_(CMP) 252, as logic highbecause clamped forward voltage V_(CF) 366, which is one possiblerepresentation of clamped forward voltage V_(CF) 166, is greater thanthe secondary threshold V_(TN). Since the inverter in FIG. 2 inverts thelogic high of compare signal U_(CMP) 352 to a logic low, the AND-gateoutputs control signal U_(CR) 364, which is one possible representationof control signal U_(CR) 164, as logic low and switch S2 127 ofsynchronous rectification circuit 126 is in OFF state. Enable signalU_(SREN) 354, which is one possible representation of enable signalU_(SREN) 254, is also logic low.

At time t₂, delayed drive signal U_(DPD) 348 goes to logic high as delaycircuit 162 generates drive signal U_(DPD) 348 in response to drivesignal U_(PD) 346. In one example, delayed drive signal U_(DPD) 348 lagsbehind the corresponding drive signal U_(PD) 346 by 100 ns. Enablesignal U_(SREN) 354 goes to logic high as delayed drive signal U_(DPD)348 sets latch 256. Primary controller 180 (responding to the transitionof delayed drive signal U_(DPD) 348 to logic high) transitions switchingdevice S1 150 to an ON state and primary current I_(SW) 330, which isone possible representation of primary current I_(SW) 130, increasesfrom zero to current limit I_(P) 132. At time t₂, clamped forwardvoltage V_(CF) 366 climbs to represents a sum of output voltage V_(O)120 plus a scaled down voltage of input V_(IN) 102. The scaled downvoltage may be substantially equal to a voltage on input V_(IN) 102scaled by the ratio of a number of turns on primary winding 110 to anumber of turns on secondary winding 112. Compare signal U_(CMP) 352 isstill greater than the secondary threshold V_(TN) which means thatcontrol signal U_(CR) 364 is logic low and switch S2 127 of synchronousrectification circuit 126 is in the OFF state.

At time t₃, primary current I_(SW) 330 reaches current limit I_(P) 132and in response, primary controller 180 switches switching device S1 150to the OFF state. Secondary winding voltage V_(S) 113 becomes a negativevoltage (which causes clamped forward voltage V_(CF) 366 to become anegative voltage as well) and the diode of synchronous rectificationcircuit 126 starts conducting current as the diode becomes forwardbiased with the negative secondary winding voltage V_(S) 113. As clampedforward voltage V_(CF) 366 goes below the secondary threshold V_(TN) ofcomparator 250, comparator 250 switches compare signal U_(CMP) 352 fromlogic high to logic low. Since the AND-gate in logic circuitry 270 hasthree logic highs on its three inputs, the AND-gate outputs controlsignal U_(CR) 364 as logic high and switch S2 127 of synchronousrectification circuit 126 transitions to the ON state.

After the sharp decline of clamped forward voltage V_(CF) 366 at timet₃, clamped forward voltage V_(CF) 366 increases as the energy in energytransfer element T1 124 is transferred from primary winding 110 tosecondary winding 112. As output voltage V_(O) 120 rises from the energytransfer to secondary winding 112, feedback signal U_(FB) 320 becomesgreater than the output threshold V_(TH).

At time t₄, clamped forward voltage V_(CF) 366 reaches the secondarythreshold V_(TN), which switches compare signal U_(CMP) 352 to logichigh. This causes the AND-gate to generate a logic low for controlsignal U_(CR) 364, which switches switch S2 127 of synchronousrectification circuit 126 to the OFF state. In response to the fallingedge of control signal U_(CR) 364 (as it transitions from logic high tologic low), one-shot circuit 258 generates a short pulse that resetslatch 256 and causes enable signal U_(SREN) 354 of the latch output togo to logic low. The short pulse generated by one-shot circuit 258 isillustrated in FIG. 3A as one-shot signal U_(ONE) 357, which is onepossible representation of one-shot signal U_(ONE) 257, in FIG. 2. Whenswitch S2 127 of synchronous rectification circuit 126 is turned OFF,clamped forward voltage V_(CF) 366 briefly drops below the secondarythreshold V_(TN) before continuing to climb. Of note, although clampedforward voltage V_(CF) 366 briefly drops below the secondary thresholdV_(TN), enable signal U_(SREN) 354 remains logic low because latch 256was reset (and has not yet been set), which ensures that control signalU_(CR) 364 does not inadvertently transition to logic high.

At time t₅, clock signal U_(CLK) 310 goes to logic high while feedbacksignal U_(FB) 320 is above the output threshold V_(TH), so drive circuit144 does not generate a logic high for drive signal U_(PD) 346. Ofcourse, without a logic high from drive signal U_(PD) 346, delayed drivesignal U_(DPD) 348 also stays logic low and primary controller 180 doesnot transition switching device S1 150 to the ON state.

At some point (right before time t₆ in FIG. 3A), feedback signal U_(FB)320 drops below the output threshold V_(TH), which causes drive circuit144 to transition drive signal U_(PD) 346 to logic high when clocksignal U_(CLK) 310 switches to logic high again (at time t₆ in FIG. 3A).With drive signal U_(PD) 346 indicating the start of an enabledswitching period, switching device S1 150 will be turned ON in responseto switch drive signal 182 by primary controller 180 and energy willonce again be delivered to the output of synchronous flyback switch modepower converter 100 to regulate output voltage V_(O) 120. Notably,following the transition of drive signal U_(PD) 346 from logic low tologic high at time t₆ with a time delay (e.g., 100 ns), delayed drivesignal U_(DPD) 348 goes to logic high at time t₇ and sets latch 256 sothat control signal U_(CR) 364 may once again go to logic high and turnON switch S2 127 of synchronous rectification circuit 126.

FIG. 3B illustrates an example timing diagram showing signals associatedwith a secondary controller (e.g., secondary controller 140) operatingin the continuous conduction mode, in accordance with the teachings ofthe present invention. The time period between time t₁ and time t₄ showsan enabled switching period, in FIG. 3B. From time period time t₁ totime t₃, FIG. 3A and FIG. 3B are similar.

In FIG. 3B, at time t₃, clamped forward voltage V_(CF) 366 drops belowthe secondary threshold V_(TN) after switching device S1 150 transitionsto the OFF state. Clamped forward voltage V_(CF) 366 then starts toincrease as the energy in energy transfer element T1 124 is transferredto the output of synchronous switch mode power converter 100. At timet₄, clock signal U_(CLK) 310 goes to logic high while clamped forwardvoltage V_(CF) 366 has not yet reached the secondary threshold V_(TN).Since clamped forward voltage V_(CF) 366 is not above the secondarythreshold V_(TN), compare signal U_(CMP) 352 stays logic low.

At time t₄, drive signal U_(PD) 346 transitions to logic high, whichindicates an enabled switching period. In response to the high drivesignal U_(PD) 346, the second input of the AND-gate goes to logic lowcausing control signal U_(CR) 364 to go to logic low as well, whichturns OFF switch S2 127 of synchronous rectification circuit 126. Thefalling edge of control signal U_(CR) 364 (which turned off switch S2127) causes one-shot circuit 258 to generate a pulse on one-shot signalU_(ONE) 357 that resets latch 256.

At time t₅, compare signal U_(CMP) 352 transitions to logic high anddrive signal U_(PD) 346 is logic low. Also at time t₅, delayed drivesignal U_(DPD) 348 transitions to logic high, which sets latch 256. Ofcourse, delayed drive signal U_(DPD) 348 also causes primary controller180 to turn ON switching device S1 150 and as a result, primary currentI_(SW) 330 begins to rise from a non-zero value, as synchronous flybackswitch mode power converter 100 is operating in the continuousconduction mode.

As discussed previously, efficiency of power converters is of highimportance in industry. For efficiency purposes, switching device S1 150and switch S2 127 of synchronous rectification circuit 126 shouldgenerally not be turned ON at the same time. In the illustrated example,secondary controller 140 is configured to control the switching ofswitch S2 127 in coordination with primary controller 180 in both thediscontinuous conduction mode (FIG. 3A) and the continuous conductionmode (FIG. 3B), ensuring that switching device S1 150 and switch S2 127of synchronous rectification circuit 126 are not turned ON at the sametime.

FIG. 4 is a flow chart illustrating an example process 400 of operationof a synchronous flyback switch mode power converter, in accordance withthe teachings of the present invention. Process 400 starts at processblock 405 with a primary switch (e.g., switching device S1 150) enabled(i.e., allowed to be turned ON). In process block 410, an input signalrepresentative of a secondary winding voltage of a secondary winding ofan energy transfer element of a synchronous flyback switch mode powerconverter is monitored. In decision block 420, the input signalrepresentative of a secondary winding voltage of a secondary winding iscompared to a threshold (e.g., the negative threshold V_(TN)). If theinput signal representative of a secondary winding voltage of asecondary winding is greater than the threshold indicating that theprimary switch is in the OFF state, process 400 returns to process block410. If the input signal representative of a secondary winding voltageof a secondary winding is less than the threshold indicating that theprimary switch has been switched to the OFF state, a secondary switch(e.g., switch S2 127 of synchronous rectification circuit 126) coupledto the secondary winding is enabled (e.g., switched to the ON state) inprocess 430.

In process block 440, the input signal representative of a secondarywinding voltage of a secondary winding is monitored. In decision block450, the input signal is compared with the threshold. If the inputsignal is greater than the threshold, process 400 proceeds to processblock 460 where the secondary switch is disabled (e.g., switched to theOFF state). This will be the process followed while the synchronousflyback switch mode power converter is in the discontinuous mode. If theinput signal representative of a secondary winding voltage is notgreater than the threshold in decision block 450, process 400 willproceed to decision block 470 and determine whether a primary switch isenabled. If the primary switch is enabled in decision block 470, process400 continues to process block 480 where the secondary switch isdisabled. This will be the process followed while the synchronousflyback switch mode power converter is in the continuous conductionmode. Thus, the secondary switch is disabled in response to at least oneof a plurality of events. The plurality of events that may triggerdisabling the secondary switch includes the input signal reaching thethreshold and includes a drive signal (e.g., drive signal U_(PD) 346)having a level (e.g., logic high) to enable the primary switch. In oneexample, the plurality of events that will cause the secondary switch tobe disabled may include the input signal reaching the threshold and thedrive signal having a level to enable the primary switch.

If in decision block 470 the primary switch is not enabled, process 400returns to process block 440. In the illustrated example, after thesecondary switch is turned OFF in process block 460 or 480, process 400ends at process block 490. However, in one example, process 400 returnsto process block 405 instead of ending at process block 490.

In one example, the drive signal is generated in response to a clocksignal and in response to a feedback signal representative of an outputquantity (e.g., output quantity U_(O) 156) of the synchronous flybackswitch mode power converter. In one example, the output quantityincludes the output voltage of the synchronous flyback switch mode powerconverter. The primary switch may be coupled to a primary winding of theenergy transfer element and control a transfer of energy through theenergy transfer element from an input of the synchronous flyback switchmode power converter to an output of the synchronous flyback switch modepower converter.

In one example, enabling the secondary switch (process block 430)includes setting a latch in response to the drive signal and disablingthe secondary switch (process block 460 or 480) includes resetting thelatch. Setting the latch may include delaying the drive signal such thatthe latch is set in response to the delayed drive signal. Resetting thelatch may include detecting a falling edge of a control signal thatenables and disables the secondary switch and the latch may be reset inresponse to detecting the falling edge of the control signal.

The above description of illustrated examples of the present invention,including what is described in the Abstract, are not intended to beexhaustive or to be limitation to the precise forms disclosed. Whilespecific embodiments of, and examples for, the invention are describedherein for illustrative purposes, various equivalent modifications arepossible without departing from the broader spirit and scope of thepresent invention. Indeed, it is appreciated that the specific examplevoltages, currents, frequencies, power range values, times, etc., areprovided for explanation purposes and that other values may also beemployed in other embodiments and examples in accordance with theteachings of the present invention.

What is claimed is:
 1. A secondary controller for use in a powerconverter, the secondary controller comprising: a drive circuitgalvanically isolated from a primary side of the power converter,wherein the drive circuit is coupled to a secondary side of the powerconverter, wherein the secondary side of the power converter isgalvanically isolated from a primary side of the power converter,wherein the drive circuit is coupled to generate a first signal on thesecondary side of the power converter to enable a first switch coupledto the primary side of the power converter, wherein the secondarycontroller is configured to provide partial control of the first switchand does not generate a turn off signal to disable the first switch, andwherein the first signal is generated in response to a feedback signalrepresentative of an output of the power converter; and a controlcircuit coupled to receive the first signal and an input signalrepresentative of a secondary winding voltage of the power converter,wherein the control circuit is coupled to generate a second signal tocontrol a second switch coupled to the secondary side of the powerconverter in response to the first signal and the input signal, whereinthe control circuit further comprises a comparator coupled to receivethe input signal and is coupled to generate a compare signal in responseto the input signal and a threshold, wherein the control circuit iscoupled to generate the second signal in response to the first signaland the compare signal, wherein the control circuit includes a latchcoupled to be set in response to the first signal and coupled to bereset in response to the second signal, and wherein the control circuitis coupled to generate the second signal in response to an output of thelatch.
 2. The secondary controller of claim 1, wherein the controlcircuit is coupled to turn on the second switch when the input signal isless than a threshold and turn off the second switch in response to atleast one of a plurality of events, wherein the plurality of eventsincludes the input signal reaching the threshold after the input signalhas fallen below the threshold and includes the first signal having alevel to enable the first switch.
 3. The secondary controller of claim1, further comprising a delay circuit coupled to receive the firstsignal and coupled to generate a delayed first signal, wherein the latchis coupled to the delay circuit to be set in response to the delayedfirst signal.
 4. The secondary controller of claim 1, wherein thecontrol circuit includes a logic gate coupled to generate the secondsignal in response to the first signal, the compare signal, and thelatch output.
 5. The secondary controller of claim 4, wherein the logicgate is an AND gate.
 6. The secondary controller of claim 1, furthercomprising a one-shot circuit coupled to the latch and coupled toreceive the second signal, wherein the one-shot circuit is coupled toreset the latch in response to the second signal.
 7. The secondarycontroller of claim 6, wherein the one-shot circuit is coupled to resetthe latch in response to a falling edge of the second signal.
 8. Thesecondary controller of claim 1, wherein the input signal is generatedby a winding sense circuit that is coupled to a secondary winding of anenergy transfer element of the power converter.
 9. The secondarycontroller of claim 1, further comprising an oscillator coupled togenerate a clock signal coupled to be received by the drive circuit,wherein the drive circuit is coupled to generate the first signal inresponse to the clock signal.
 10. The secondary controller of claim 1,wherein the second switch is a synchronous rectifier.
 11. The secondarycontroller of claim 1, wherein the first switch coupled to the primaryside of the power converter is configured to be disabled by a primarycontroller coupled to the primary side of the power converter andenabled by the secondary controller.
 12. A method of controlling a powerconverter, the method comprising: generating a first signal on asecondary side of the power converter to enable a first switch coupledto a primary winding of an energy transfer element to enable a transferof energy through the energy transfer element from an input of the powerconverter to an output of the power converter, wherein the secondaryside of the power converter is galvanically isolated from a primary sideof the power converter, wherein the first signal is generated in asecondary controller galvanically isolated from the first switch, andwherein the secondary controller is configured to provide partialcontrol of the first switch and does not generate a turn off signal todisable the first switch; comparing a threshold to an input signalrepresentative of a voltage of a secondary winding of the energytransfer element; generating, in the secondary controller galvanicallyisolated from the first switch, a second signal to enable a secondswitch coupled to the secondary winding in response to said comparingthe threshold to the input signal; and generating the second signal todisable the second switch in response to at least one of a plurality ofevents, wherein the plurality of events includes the input signalreaching the threshold and includes the first signal having a level toenable the first switch, wherein generating the second signal to enablethe second switch comprises setting a latch in response to the firstsignal, and wherein generating the second signal to disable the secondswitch comprises resetting the latch.
 13. The method of claim 12,wherein setting the latch comprises delaying the first signal such thatthe latch is set in response to the delayed first signal.
 14. The methodof claim 12, wherein resetting the latch comprises detecting a fallingedge of a second signal that enables and disables the second switch,wherein the latch is reset in response to said detecting the fallingedge of the second signal.
 15. The method of claim 12, furthercomprising generating the first signal in response to a clock signal andin response to an output of the power converter.
 16. The method of claim12, wherein the first switch coupled to the primary winding of theenergy transfer element is configured to be disabled by a primarycontroller coupled to the primary side of the power converter.